1. Field of the Invention
This invention relates to a radiation image storage panel for use in radiation image recording and reproducing techniques utilizing stimulable phosphors.
2. Description of the Related Art
Radiation image recording and reproducing systems utilizing stimulable phosphors have heretofore been known as computed radiography (CR). The radiation image recording and reproducing systems utilizing the stimulable phosphors comprise a radiation image recording apparatus for irradiating radiation carrying image information of an object, such as a human body, and storing a radiation image of the object as a latent image on a layer of the stimulable phosphor. The radiation image recording and reproducing systems utilizing the stimulable phosphors also comprise a radiation image read-out apparatus for exposing the stimulable phosphor layer, on which the latent image of the radiation image has been stored, to stimulating rays, such as a laser beam, which cause the stimulable phosphor layer to emit light in proportion to the amount of energy stored on the stimulable phosphor layer during the exposure of the stimulable phosphor layer to the radiation, and photoelectrically detecting the emitted light in order to acquire an image signal representing the radiation image of the object.
As recording media utilized in the aforesaid radiation image recording and reproducing systems, radiation image storage panels comprising, for example, a substrate having rigidity and a stimulable phosphor layer, which are overlaid one upon another in a layer form, have been known. After image signals have been detected from the radiation image storage panels, the radiation image storage panels are exposed to erasing light, and energy remaining on the stimulable phosphor layers is thus erased. The erased radiation image storage panels are then capable of being used again for the recording of radiation images. In this manner, the radiation image storage panels are capable of being used repeatedly for the recording and readout of radiation images. Ordinarily, such that chemical deterioration of the stimulable phosphor layers or physical impacts to the stimulable phosphor layers due to the repeated use of the radiation image storage panels maybe suppressed, a protective layer is overlaid on the surface of the stimulable phosphor layer, which surface is opposite to the surface that stands facing the substrate.
For example, Japanese Unexamined Patent Publication No. 2001-141896 discloses a radiation image storage panel comprising a fluorine resin film, which has a thickness falling within the range of 1 μto 10 μm and has a transmittance of at least 90% with respect to light having wavelengths falling within the range of 400 nm to 900 nm and which acts as a protective layer, and a stimulable phosphor layer adhered to the fluorine resin film by use of an adhesive agent. The protective layer constituted of the fluorine resin film has good stainproof characteristics and good water vapor proof characteristics and is a thin film. Therefore, the disclosed radiation image storage panel is capable of yielding an image of good image quality.
Also, a radiation image storage panel, which comprises a protective layer imparted with water vapor proof characteristics, is proposed in, for example, Japanese Unexamined Patent Publication No. 2002-98799. The proposed radiation image storage panel comprises a protective layer, which is formed with a vacuum evaporation process and contains aluminum oxide and/or a silicon oxide. The protective layer has a thickness falling within the range of 0.01 μm to 0.5 μm and does not absorb light having wavelengths falling within the range of 300 nm to 1.000 nm. Also, the protective layer has a water vapor transmission rate of at most 2 g/m2/24 h and an oxygen transmission rate of at most 2 cc/m2/24 h. The protective layer thus has gas barrier characteristics. The protective layer employed in the proposed radiation image storage panel, which is protective layer is constituted of the inorganic substance evaporated layer having the gas barrier characteristics, is capable of being formed as a thin layer, which is free from blurring of image quality due to light scattering. Therefore, the proposed radiation image storage panel is capable of yielding an image having good initial image quality.
Further, a radiation image storage panel having a high water vapor resistance is described in, for example, U.S. Patent Laid-Open No. 20030038249. The radiation image storage panel described in U.S. patent Laid-Open No. 20030038249 comprises two protective layers having different water vapor transmission coefficients (water vapor transmission rates).
The radiation image storage panel disclosed in Japanese Unexamined Patent Publication No. 2001-141896, which comprises the thin fluorine resin film, is capable of yielding an image having good image quality and has good water vapor proof characteristics. However, for technical and economical reasons, it is not always possible to form a thin and uniform fluorine resin film. Therefore, with the radiation image storage panel disclosed in Japanese Unexamined Patent Publication No. 2001-141896, the problems are encountered in that image nonuniformity is apt to occur.
The protective layer (the transparent inorganic layer) employed in the radiation image storage panel proposed in Japanese Unexamined Patent Publication No. 2002-98799 is formed with the vacuum deposition technique. However, in cases where film forming is performed with the vacuum deposition technique on a protective layer substrate (a PET film) or an organic layer, on which the inorganic layer is to be formed, contraction and wrinkles occur with the film. Therefore, an inorganic layer having sufficient water vapor proof performance is not capable of being obtained. Also, the problems occur in that the image quality of the image obtained with the radiation image storage panel is adversely affected by the contraction and the wrinkles occurring with the film. Further, with respect to a stimulable phosphor layer, which is constituted of pillar-shaped crystals and has a gap structure, it is not always possible to form a uniform inorganic evaporated layer.
The protective layer employed in the radiation image storage panel described in U.S. patent Laid-Open No. 20030038249 has a water vapor resistance higher than the water vapor resistances of conventional radiation image storage panels. However, the protective layer employed in the radiation image storage panel described in U.S. patent Laid-Open No. 20030038249 is apt to become thick. Therefore, with the radiation image storage panel described in U.S. Patent Laid-Open No. 20030038249, the image quality of the obtained image will be apt to become bad.